The life cycle of Plasmodium falciparum is one of the most complex life cycles of any organism. The complete life cycle involves both intracellular and extracellular stages in humans, as well as in the mosquito.
Infection of a new human host is initiated when a carrier mosquito takes a blood meal. During the process of probing for food, saliva containing Sporoziotes in the mosquito's saliva is injected into the human host. While some of these sporoziotes enter the bloodstream, recent work has shown that a majority of them are deposited into the dermis and not directly into the blood. Within three hours following injection into the skin, most of the sporozoites leave the injection site via the bloodstream, the lymphatic system or direct migration through tissue. Regardless of the route taken, the sporozoite must migrate through cell barriers in both the skin and in the ultimate target organ, the liver. To this end, it has recently been shown that Plasmodium sporozoites are capable of traversing cells without initiating replication within the cell.
Once the sporozoites are in the bloodstream, they rapidly localize to the liver. This preference for liver tissue is mediated by an interaction between the major surface protein of sporozoites, the circumsporozoite protein (CSP), and highly sulfated proteoglycans present in loose, basement membrane of the liver. Once they have reached the liver, it appears that sporozoites actively invade Kupffer cells using a process involving gliding motility. This process, which does not involve flagella, involves interactions between parasite membrane proteins and an extracellular, polysaccharide substrates secreted by the parasite. Additionally, cell invasion involves the ordered release of proteins and other molecules from secretory organelles, called micronemes and rhoptries, present at the apical end of the zoite. The sporozoite then traverse the invaded cell, crossing into the space of Disse, and further migrating through several other hepatocytes. The sporozoite then invades a final hepatocyte, with the parasite forming an encapsulating structure referred to as a parasitophorous vacuole (PV). At this point, the organism begins liver stage (LS) growth.
Little is known about growth of the organism during the liver stage. What is known, however, is that following formation of the PV, the sporozoite differentiates into a liver trophozoite. Following this differentiation, growth and asexual replication through a process known as exoerythrocytic schizogony is rapid, requiring the ability to obtain nutrients from the host, as well as the ability to cause an increase in cell volume without damaging the host cell. This latter ability is related to the parasite's ability to confer resistance to apoptosis of the host cell. This stage of the life cycle culminates in the production of mreozoites, which are released into the blood.
Once in the blood, the parasites begin the blood stage, or erythrocytic stage of the life cycle. It is this stage of the infection that results in the pathology associated with malaria. This stage is also referred to as the ring stage, due to the ring-like morphology of the early trophozoite. During this phase, the merozoites invade erythrocytes and undergo a trophic period in which the parasite enlarges. Enlargement of the trophozoite results in active metabolism, which includes ingestion of host cytoplasm and proteolysis of hemoglobin. The trophic phase ends with multiple rounds of nuclear division resulting in the formation of schizonts. These schizonts then bud off merozoites, which are released upon rupture of the infected erythrocyte. Once released, the merozoites infect new erythrocytes, and begin another round of blood-stage replication.
During subsequent rounds of replication, some parasites switch from an asexual replication strategy to a sexual replication strategy. In this reproduction strategy, some parasites differentiate into either macrogametocytes or microgametocytes. These gametocytes, which contain a single nucleus, are large parasites that fill the entire erythrocyte. Following their release into the blood, the gametocytes are ingested by mosquitoes during the taking of a blood meal. This begins what is referred to as the sporogonic cycle.
Once in the mosquitoe's stomach, the microgametes penetrate the macrogametes, resulting tin the formation of zygotes. Soon after zygote formation, meiosis occurs and the spherical zygote transforms into an elongated motile cell called an ookinete. The ookinete uses its motile ability to penetrate the matrix surrounding the blood meal, and traverse several layers of epithelial cells before exiting through the basal side of the epithelium. Upon reaching the basal surface, the ookinete begins its transformation into an oocyst.
Maturation of the oocyst is a long process, taking approximately 12 days. During this process, the oocyst grows in size, eventually becoming 50-60 μm in diameter. At some point during this phase, sporozoites are produced. These sporozoites are mobile, although this motility is still immature. Eventually, the sporozoites are released from the oocyst, due in large part to the action of a cysteine protease called the egress cysteine protease (ECP1).
Once released, the sporozoites are carried to all tissues of the mosquito by the circulating hemolymph. Upon reaching the basal lamina of the salivary glands, ligands on the outer surface of the sporozoite interact with receptors, allowing the sporozoite to adhere to the basal surface. The parasites move through the basal lamina and invade the salivary gland acinar cells. This invasion is mediated by a short-lived vacuole that transports the sporozoite through the cytoplasm of the acinar cells, and out through the apical surface. The process ends with the sporozoite ending up in the salivary gland duct. Thus, the sporozoites are ready to infect a new host during the next blood meal.
According to the Centers for Disease Control, malaria is one of the most severe public health problems worldwide, it is the leading cause of death and disease in many developing countries, affecting mostly young children and pregnant women. Malaria is the cause of at least one million deaths every year, with 350 and 500 million clinical episodes occurring every year. More than 80% of the malaria deaths worldwide occur in Africa south of the Sahara. Currently there is no vaccine available, and there is growing resistance to existing anti-malarial drugs. Only one drug (primaquine) is used to kill non-erythrocytic stages (the gametocyte and liver stages), it has serious side effects and the concern of resistance to the only drug that can kill non-erythrocytic stages prevents wide use of this drug. Thus, there is an urgent need for new malarial drugs and particularly drugs that can effectively treat non-erythrocytic stages thereby disrupting the infectious cycle of the infective organisms to prevent transmission between individuals during the non-infectious gametocyte and liver stages and to eradicate the infection in an individual before the erythrocytic stage develops.